Technical Field
The present invention relate to a friction material, and more specifically a friction material to be used for brake pads, brake linings, clutch facings, etc. of industrial machines, railway vehicles, cargo vehicles, passenger automobiles, etc.
Background Art
Friction materials are used for brakes such as disc brakes and drum brakes, clutches or the like, and play a role in braking by friction with counterpart materials to the disc brakes, etc. Properties required for the friction materials include, for example, a high friction coefficient, a small decrease in friction coefficient at high load, high temperature or the like (fading properties), low possibility of generation of noise or squeal in braking, low counterpart material attack, etc.
The friction material is formed of raw materials such as a fiber base material for exerting a reinforcing function, a friction modifying material for imparting a friction function and modifying its friction performance and a binder for integrating these components. Examples of materials for modifying friction properties of the friction material include, for example, inorganic friction modifying materials such as alumina, silica, magnesia and zirconia, organic friction modifying materials such as synthetic rubbers and cashew resins, solid lubricants such as graphite and molybdenum disulfide, etc.
Alumina has been widely used for improving braking properties of the friction material. For example, Patent Document 1 proposes a friction material containing, based on the total amount of a friction material composition, 0.2 wt. % to 5.0 wt. % of calcined alumina particles containing 70 wt. % to 95 wt. % of α-alumina and the balance being transition alumina with inevitable impurities. Further, Patent Document 2 describes that in a friction material formed by hot-pressing a friction material composition containing active alumina as an inorganic friction modifying agent and a fluorine-based polymer as an organic friction modifying agent, γ-alumina is used as the active alumina. In addition, Patent Document 3 proposes a friction material composition containing α-alumina and γ-alumina in an weight ratio of α-alumina:γ-alumina=1:20 to 1:5 as a part of an inorganic friction modifying material and using a silicone-containing phenol resin as the whole or a part of a binder, and a friction material formed by hot-pressing the friction material composition.
On the other hand, the friction coefficient changes due to changes in the humidity environment, sometimes resulting in unstable friction properties. It has therefore been desired to stabilize the friction properties under the changes in humidity environment. For example, Patent Document 4 proposes, as a friction material in which the friction coefficient does not largely change due to humidity, a friction material including a non-whisker-shaped titanate compound and bio-soluble inorganic fiber.
Patent Document 1: JP-A-2013-163714
Patent Document 2: JP-A-2011-17016
Patent Document 3: JP-A-2004-346179
Patent Document 4: JP-A-2007-277418